How to disable or customize admin bar in back-end

To disable (or in the case which I show below with code, hiding) the adminbar in the back-end you could use following code snippet.

Please always backup functions.php before editing/adding code.

I added also a capability so the adminbar is still visible for the admin.
(which you can remove if wish by removing !current_user_can( 'manage_options' ) && or whatever you want to do with it (look here for other roles/capabilities, to understand all a little more) )

/**
 * Hide the Toolbar(adminbar) in the back-end using CSS
 * 
 * @version WP 4.8
 * Read more {@link https://codex.wordpress.org/Roles_and_Capabilities#Capabilities}
 *           {@link https://codex.wordpress.org/Function_Reference/is_admin}
 *           {@link https://codex.wordpress.org/Function_Reference/current_user_can}
 */
if ( !current_user_can( 'manage_options' ) && is_admin() )
{
    function wpse271937_hide_adminbar()
    {
        ?>
        <style>
            #wpadminbar {
                display: none;
            }
            #wpwrap {
                top: -30px;/** change to own preference */
            } 
        </style>
        <?php
    }
    add_action('admin_head', 'wpse271937_hide_adminbar'); 
}

To customize the adminbar I leave that to your own preferences, but you can for sure take a look here or maybe in the Codex itself.

Take also a look at these:

• wp_before_admin_bar_render
• admin_head (plugin page)

I hope this helps you on your way.

Note: the Screen Options and Help tab are with the function as shown above still visible which imho is important. Of course is it also possible to hide/remove them with a (seperate) function.